I replaced the transistors, applied the thermal paste, and replaced the faulty resistor R651. Powering on with the DBT has the tester bulb dimly lit, and the cabinet light bulbs also dimly lit. The input LEDs are lit, and they change when turning the input select knob. The safety LED is not lit. No sound is coming from the headphone jack, even with the FM muting switch off, or in AM mode. Even with this, I think we're close. I hope you have a Merry Christmas!
I used some contact cleaner on the power display switch, after working it some, the green LED in the center now lights up.
I did the same to the REC MODE/ TAPE MONITOR switches, and fully screwed in the front panel into the chassis.
Now, the DBT bulb glows brighter and the chassis bulb glows dimmer.
Replacing the bottom cover has eliminated the buzzing. No sound remains after powering on.
I did the same to the REC MODE/ TAPE MONITOR switches, and fully screwed in the front panel into the chassis.
Now, the DBT bulb glows brighter and the chassis bulb glows dimmer.
Replacing the bottom cover has eliminated the buzzing. No sound remains after powering on.
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I decided to try testing R908 and D902. R908 read the correct resistance of 2.2kOhm, and D902 showed the same bias in either direction. Since D902 is a zener diode, I'm not sure if that's expected or not. I also took voltage measurements at the cathode of D902, it showed -14V whereas the schematic showed that the expected voltage is -2.5V. I think D902 may be shorted.
I found this thread that has a similar issue to me: Technics SA600 Relay not engaging. | Audiokarma Home Audio Stereo Discussion Forums
I measured the values of the components listed in the thread, but didn't find anything unusual in the component values themselves. The voltages at the diodes and transistors are incorrect, however.
I also removed and inspected the relay. It seems to be fine, though there was a bit of that green gel on one of the legs. I cleaned it off and replaced it, but it still doesn't work.
I measured the values of the components listed in the thread, but didn't find anything unusual in the component values themselves. The voltages at the diodes and transistors are incorrect, however.
I also removed and inspected the relay. It seems to be fine, though there was a bit of that green gel on one of the legs. I cleaned it off and replaced it, but it still doesn't work.
Ok, never mind. After testing the transistors out of the circuit, they're fine. But I took another look at C904. It is listed in the parts list as ECEA16N220V, and ECE***N means non-polar electrolytic. The capacitor that I replaced it with (from a kit I got off ebay) was a polar electrolytic 25V 220uF capacitor. I'm willing to bet that's the problem. I hadn't put that together with the circled 'N' on the schematic. I'll order a replacement and see if that corrects the issue. Guess that shows me for blindly trusting a kit off ebay!
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I replaced C904 with a nonpolar electrolytic capacitor. No change. I noticed that the polarization on c903 is different between the physical schematic/ PCB marking and the circuit diagram. The board is marked such that the negative lead is connected to the base of Q904, whereas on the circuit diagram, the negative lead is connected to ground. I tried swapping the capacitor, but no change. I don't know whats next. I've been making voltage readings on every lead of each component of this section of the board to try and see if I can find something. But I'm not really sure.
The main problem was severe 60 Hz hum. I found that the chassis felt strange, and testing against ground, the chassis is at around 30 VAC. I attached the ground post on the rear panel to ground, and the chassis is now grounded. The hum remains. I did some searching and found that the likely cause was a ground loop. I measured the continuity between chassis and expected ground points throughout the circuit board. Two major points are seperated from ground: the negative lead of C708, and pin 40 from the transformer. In both cases, I think the screw post that connects the board to the chassis should be electrically connected to the chassis, but it isn't. By soldering wires from those isolated ground points to other grounds that are continuous to the chassis, I was abale to significantly reduce the hum. The phono and aux imputs work great, but the radio doesn't function, just static. I'm thinking it needs an alignment. I'll check with my local community college to see if I can borrow some equipment. But this is significant progress! Thank you @mbz for your help!